Under certain conditions of temperature and mechanical stresses, certain materials such as titanium, titanium alloys, aluminium or certain of its alloys, certain steels, etc., exhibit superplasticity, i.e. the capacity to undergo a large amount of deformation without rupture. This property makes it possible to manufacture components of complex shape by a process of superplastic moulding commonly referred to by the acronym SPF (SuperPlastic Forming).
Metal moulds are already known in the current state of the art for the shaping of components by superplastic forming. Such metal moulds are relatively costly as they are fabricated in special alloys and require complex machining operations. Furthermore, the metal moulds consume a large quantity of energy for heating to a temperature suitable for thermoplastic moulding and are sensitive to uneven temperature distribution and to temperature variations which can lead to deformation of the moulds.
In order to remedy these drawbacks as far as possible, a mould has been proposed in the current state of the art, notably in U.S. Pat. No. 4,984,348, U.S. Pat. No. 5,661,992 or U.S. Pat. No. 5,214,949, for the shaping of a component in titanium or titanium alloy by superplastic forming. The mould normally comprises a base in which a moulding cavity is formed, and a cover, between which a titanium or titanium alloy plate is designed to be placed. In accordance with a conventional process, the mould is heated, the plate is clamped between the base and the cover, then an inert gas is injected under pressure between the cover and plate. Under the effect of the gas pressure, the plate undergoes superplastic deformation and assumes the shape of the mould cavity.
U.S. Pat. No. 4,984,348, U.S. Pat. No. 5,661,992 and U.S. Pat. No. 5,214,949 describe moulds fabricated at least partially in ceramic. This material is more particularly a refractory concrete generally consisting of a filler based on granular vitreous silica and a binder based on aluminate or silicate.
In a refractory concrete, the binder forms a matrix within which the granular filler is held. However, under certain conditions, the grains of the granular filler are capable of being separated from the matrix. In particular, a material such as titanium or a titanium alloy brought to a superplastic state in a refractory concrete mould enters the microcavities in the surface of the mould in contact with the material being moulded. Upon demoulding of the formed items, this leads to separation of material at the surface of the mould and/or to defects at the surface of the formed items. In addition, the mould suffers premature wear. These drawbacks result in numerous moulded components being rejected.
Furthermore, under conditions of superplastic moulding, the materials forming the binder of the refractory concrete of which the mould is made, such as aluminates or silicates, tend to migrate into the moulded component to a depth that may reach several microns. Such surface contamination of the moulded component is not acceptable in certain applications, notably in the case of moulded components in titanium or titanium alloy intended for use in the aircraft industry.